The present invention is directed to the field of the silicon doping of GaAs and (Al,Ga)As and other 3-5, 4-6 and 2-6 compound semiconductors during molecular beam epitaxy (MBE) growth, and particularly to the use of ionized silicon doping.
In the manufacture of modulation-doped field effect transistors (MODFETs) using molecular beam epitaxy technology with GaAs and (Al,Ga)As heterostructures, some of the items to be controlled are the MODFET threshold voltage, reproducibility and uniformity over large substrate areas, and minimizing of MBE surface defects. Uniformity of thickness and doping is very important since the FET threshold voltage depends on the Nd.sup.2 product, where N is the doping and d is the thickness of the doped (Al,Ga)As layer.
The conventional past approach to dope (Al,Ga)As and GaAs n-type during molecular beam epitaxy (MBE) growth is by a neutral thermal molecular beam of silicon from an effusion cell. A limitation of this approach is that the flux of neutral silicon is very low (&lt;10.sup.11 molecules/cm.sup.2 -sec) and is not easily measurable by an ion gauge. Thus the reproducibility of doping with neutral silicon is difficult.
This invention replaces the neutral Si beam with a low energy silicon ion (Si.sup.+) beam. The flux of Si ions can be measured with an ion probe at the substrate location and thus the reproducibility of the doping is improved as the silicon flux is directly measured.